Equine Herpesvirus (EHV)

Equine herpesvirus (EHV) is a family of DNA viruses (viruses where the genetic material is made up of DNA) that affect horses.

What Is Equine Herpesvirus?

The most common strains that impact horses in North America are EHV 1 and EHV 4.

• Equine EHV 1 most commonly causes respiratory illness, but can also cause pregnancy loss or neurologic disease.

• Equine EHV 2 can appear in horses of all ages. Its significance is currently unclear.

• Equine EHV 3 causes equine coital exanthema, which is a highly contagious sexually transmitted disease.

• Equine EHV 4 is mainly a respiratory disease, but in rare cases may cause pregnancy loss or neurologic symptoms as well.

• Equine EHV 5 is associated with equine multinodular pulmonary fibrosis. The role of EHV 5 leading to EMPF is unclear. Clinical signs include weight loss, cough, fever and respiratory difficulty.

Equine herpesvirus is endemic in the horse population, meaning that most horses have had the disease or have been exposed to it. Mild cases of EHV in horses may even go unnoticed.

Noticeable disease is most often seen in either weanlings between 4 and 12 months of age (commonly referred to as “foal snots”) or 2- and-3-year-olds in racing or training circuits.

Depending on what state you’re in, EHV in horses is a reportable disease. This means your veterinarian needs to let the United States Department of Agriculture (USDA) know that they have diagnosed a case of equine EHV.

EHV can be highly contagious and appropriate control and prevention measures must be taken.

If you suspect your horse has EHV, immediately quarantine as they can spread this illness through respiratory droplets that can pass in the air. Sanitize any riding or grooming equipment and do not share between horses. Do not share any water buckets or food buckets between horses either.

Most cases of EHV consist of mild respiratory symptoms that are not an emergency. But if you see any difficulty in breathing, neurologic signs or pregnancy loss then these may warrant an emergency visit from your veterinarian.

Equine Herpesvirus Symptoms

EHV in horses may be identified by the following symptoms and forms:

1. Respiratory

   • Fever (over 102 F)

   • Coughing

   • Depression

   • Inappetence (not eating)

   • Nasal discharge

2. Neurologic

   • Incoordination

   • Hind limb paralysis

   • Lying down and not being able to get up (down horse)

   • Loss of bladder and tail function

   • Loss of sensation to the skin around the tail and hindlimb areas

3. Reproductive

   • Pregnancy loss during the last four months of gestation

How Is Equine Herpesvirus Spread?

Equine herpesvirus spreads when horses come into direct contact (nose-to-nose) or indirect contact (through water or feed buckets, grooming supplies, etc.) with another horse that is actively shedding the virus, most commonly through respiratory droplets.

People may inadvertently transfer the EHV if they are in contact with a horse shedding it and then interact closely with another horse, so it’s important to follow safety guidelines when caring for a sick horse.

Additionally, a horse shedding EHV may spread it by snorting or coughing. Airborne virus particles may travel a short distance, such as to a neighboring stall. 

Foals can be infected through the placenta of an infected mare. A newborn foal will often have respiratory signs that progress to a bacterial infection with a poor prognosis.

Symptoms of the EHV in horses typically develop two to five days after exposure. Infected horses can shed the virus for up to 14 days.

Diagnosing Equine EHV

Equine herpesvirus is diagnosed by testing nasal secretions and blood. Your veterinarian will take both a nasal swab and blood sample to submit for analysis.

Equine Herpesvirus Treatment

Treatment for EHV in horses is based on the form of the disease and its severity. For uncomplicated cases of respiratory disease, medications to reduce fever, and anti-inflammatories like phenylbutazone and flunixin meglumine may be all that’s required.

The goal of treatment is to keep the horse comfortable, hydrated, and still eating. Antibiotics may be added to treat bacterial infections that tend to occur secondary to equine herpesvirus. If the horse becomes dehydrated, intravenous (IV) fluids may be administered.

In severe cases, and especially neurologic cases, hospitalization may be required. Horses that cannot remain standing will require intensive care.

Recovery and Management of Equine Herpesvirus

Recovery from uncomplicated respiratory infections of EHV in horses is common and will occur in a few weeks.

The prognosis varies for horses with the neurologic form of EHV. Even with intensive care, recovery is not always possible.

Horses that recover from EHV should be rested and gradually return to work.

Once a horse recovers from equine herpesvirus, it stays in their body in an inactive state. EHV in horses can become reactivated, especially during times of stress or if the horse received steroids that suppress the immune system.

The neurologic form of the disease is often seen in older horses when the virus is reactivated.

How To Prevent Equine Herpesvirus

Vaccines are available to help prevent EHV in horses. They are aimed at limiting the respiratory and abortive forms of the disease. Though not 100% preventative, an EHV vaccine for horses may reduce the severity and duration of disease.

No vaccines are labeled to prevent the neurologic form of equine herpesvirus. Vaccination recommendations are based on the age and lifestyle of the horse.

• Vaccines are administered to broodmares at five, seven, and nine months of gestation.

• Foals should receive a three-vaccine series starting at 5 to 6 months old.

• Horses that are less than 5 years old, live in close proximity to broodmares, or travel often for racing or shows should be vaccinated every six months.

In addition to strong vaccination protocols, biosecurity, and hygiene measures should be put into place to decrease the spread of the virus.

• New horses to a facility should be isolated for three to four weeks.

• Caretakers going between horses should wash their hands and clean their clothes and shoes.

• Cleaning horse equipment and horse housing areas with a disinfectant is also recommended. A 1:10 bleach solution will inactivate the virus.

It’s always important to be cautious at horse shows and other events. This includes preventing nose-to-nose contact with other horses, not sharing water buckets or any equipment, and avoiding or cleaning areas like wash racks or hitching posts before and after use.